Microelectronic devices including frequency selective components are important for many electronic products requiring stable frequency signals or ability to discriminate between signals based on frequency diversity. These functions are difficult to reliably and inexpensively realize together with other circuitry in monolithic form on a silicon substrate.
One approach to providing microelectronic devices with frequency selective functions employs a mass allowed to vibrate in one or more dimensions (e.g., a pendulum). Such a mass is conveniently realized as a thin film supported at critical points, for example, peripherally or alternatively along one edge or end, forming a thin-film resonator structure. The term "thin film" in this context refers to a material deposited using, for example, chemical vapor deposition, evaporation, sputtering or other integrated circuit chip fabrication techniques, and having a thickness in the range of from about a few (2-3) atomic layers to about a few (1-5) microns, with typical thicknesses being about 2-4 microns. Such thin-film structures provide clearly defined mechanical resonances having significant utility, for example as filters in cellular phones and other communications devices or as frequency stabilizing feedback elements in oscillator circuits.
A significant drawback to such suspended mass resonators has been the need to fabricate the free-standing thin-film membrane. Typically, this is done by depositing the thin-film membrane over a sacrificial layer, then selectively removing the sacrificial layer to leave the thin film self-supported. Alternatively, the substrate is etched from the back to provide an opening extending up to the bottom of the membrane. Another approach is to form a cantilevered beam, capacitively coupled to adjacent structures (e.g., by means of a conductor placed beneath the beam). The beam is free to vibrate and has one or more resonance frequencies. Disadvantages of these approaches include the need to form free-standing structures and also a tendency of the beam to "stick" to adjacent structures if or when the beam comes into contact therewith. A need to remove any sacrificial layer and/or underlying substrate material limits fabrication ease and results in structures which are extremely fragile with respect to externally applied forces. These factors contribute to reduced fabrication yields and reduced robustness of the finished resonator component.
Dworsky et al., U.S. Pat. No. 5,373,268, the totality of which is incorporated herein by reference, describes a thin-film resonator having solid mechanical support. In the '268 patent, a piezoelectric element, sandwiched between two electrodes, is supported on an acoustic reflector which presents a high acoustic impedance, analogous to a clamped surface, when situated atop a low acoustic impedance substrate. The reflector has an odd number of alternating layers of high and low acoustic impedance materials of one-quarter wavelength thickness that act to exhibit the transmission line Ferranti effect, whereby the low impedance of the substrate at one end of the reflector transmission line is transformed to a high impedance at the other end closest to the bottom electrode. This enables a one-quarter wavelength thickness resonator to be supported atop the reflector with its bottom electrode effectively "clamped" at the high acoustic impedance end of the reflector. The '268 patent mentions that the same impedance transformation effect can be realized using tapered or other tailored impedance profiles.
Thin-film resonators (and filters based on the use of thin-film resonators) require packaging similar to that used for crystal resonators. In a crystal resonator element, a plate of piezoelectric crystal is suspended inside a hermetic package. Even though both surfaces are metallized, the crystal quality factor and frequency are very sensitive to particulates and changes in humidity. Likewise, thin-film resonators, including those described in the '268 patent which have a reflector structure for mounting on a substrate, require environmental isolation from particulates and humidity. To achieve this isolation, present thin-film resonators must be packaged in high-cost, hermetically sealed metal packages, evacuated or back-filled with inert gas. Inexpensive plastic molding compounds used for packaging integrated circuit dies (viz. compounds with low acoustic impedance and thicknesses greater than one wavelength) are not usable for encapsulating thin-film resonators because they dampen vibration and interfere with acoustic operation. Moreover, to avoid interference with acoustic operation, present-day resonators are not even coated with chemically vapor deposited silicon oxide or silicon nitride, spin-on glass, spin-on polyimide, or similar passivation insulator films (low acoustic impedance and typical thicknesses of 500 to 4,000 Angstroms) deposited as a last line of defense against moisture and other contaminants for integrated circuits.
There is a need for inexpensive microelectronic devices including frequency selective components of the thin-film acoustic resonator type. Thus, there is also a need for low cost environmental isolation and packaging for thin-film resonators and thin-film resonator-based filters.